Gels comprising silica



t 1945- M. M. MARISIC ET AL GELS COMPRISING SILICA Filed April 8, 1943W4 TE m M R RM Y w M yo N0 T N L T M 6 MY .B

Patented Oct. 16, 1945 assignors to Socony-Vacuum Oil Company,Incorporated, acorporation of New York 14 Claims.

This invention is concerned with the preparation of hydrogels comprisingsilica and is more particularly directed to an improvement in previouslyknown processes whereby hydrogels of greater oxide content may beprepared which can then be dried to give rigid solids.

In the preparation of synthetic porous solids comprising silica havingadsorbent properties, hydrated silica. or silicic acid is caused toseparate from an aqueous solution. Under proper condi tions, the silicawill be in the form of a true gel, but under other conditions, agelatinous precipitate of silicic acid is formed. In some cases, it is amatter of little importance whether a gel or precipitate is formed, andthe precipitate has that it is not properly a gel at all. In many cases,however, it is highly desirable that a true gel be formed. For example,the process of forming spheroidal particles described in the copendingapplication Serial No. 481,454, filed October 9, 1942, by Milton M.Marisic, it is essential that there be gelation with substantially noprecipitation. The Marisic process is described in detail below inconsidering preferred embodiments of the present invention. Briefly, theMarisic proc ess involves formation of gelable colloidal solution,separation of the solution into globules and gelation in the globularform. A suitable process is to project a stream of freshly preparedcolloidal solution into a body of oil in which the stream breaks up intoglobules which gel as they fall through the oil. The gel globules arethen washed and dried to give a mass of solid porous beads. It isapparent that gelation time is an important factor in such a process.Certain general relationships have been found in this connection. Thetime of gelation is dependent upon temperature, pH of the colloidalsolution and concentration of gelable substance. The higher thetemperature. the shorter the time of gelation. At fixed concentration ofreactants, the gelation time increases with decrease in pH. When thetemperature and pH are constant, the gelation time decreases as thereactant solutions are made more concentrated. It has been found thatthe preferred gels for many purposes, notably when the gel is intendedas catalyst for cracking of hydrocarbons, are prepared from colloidalsolutions of pH to 8, and further that gels of commercial value may beprepared from colloidal solutions of pH 2.5 to about pH 10. Further, ifthe concentration of any given type of colloidal solution is increasedabove a critical value, precipitation takes place. It follows that theknown facts im- Application April 8, 1943, Serial No. 482,331

- often been referred to as a gel, despite the fact pose a limitingvalue on concentration of solids in the gel. In preferred embodiments ofthe dried gel in the form of spheroidal bodies having smooth hardsurfaces, a gelation time suflicient to permit extrusion into 011 beforegelation is essential and in any gelation process the concentration mustbe low enough to permit gel formation.

Yet, from the economic standpoint, it is desirable to prepare a gel ofas great a solid concentration as possible. For example, high solidconcentration reduces the load on drying equipment. High concentrationgives a greater amount of active material per unit volume of hydrogeland, accordingly, increases the capacity of the gel forming units. Bythe methods of this inven tion, dried gels may be manufactured whichhave higher densities than other contact materials used for similarpurposes as knownin the arts. Increased density is itself desirable insome cases. For example, if small particles, say, 0.1 mm. are preparedfor use in the so-called fluid catalyst" process, increased densitymakes for emcient separation of catalyst from gases. In that process acatalyst of small sized particles is suspended in a stream of gas and,after treatment is complete, the solid is separated as in a cycloneseparator. Obviously, particles of greater density tend to separate morereadily from the gaseous suspending medium.

We have now found that the concentration of the hydrogel may besubstantially increased by using a combination of a strong acid and aweak acid to adjust the pH of a stable aqueous alkaline solution ofinorganic salts to a gelation pH. For example, silica gel is preparedaccording to the prior art by acidifying a water glass solution. If theacidifying agent is a combination of a weak and a strong acid, greaterconcentration of silica is possible without precipitation and theconcentration of silica may even be increased with an increased gelationtime as compared with solutions prepared by the use of strong acids onlyin the conventional manner. The term strong acid" is used here in itsnormal sense, referring to the relatively few mineral acids which aresubstantially completely ionized in moderately concentrated (normal)aqueous solution. As is well known, this group includes hydrochloric,sulfuric and nitric acids. In contrast, all, acids not falling withinthis group are known as weak acids. The weak acid seems to stabilize thehighly concentrated silica sol and inhibits precipitation, so that thesol sets to a hydrogel without appreciable precipitation.

The relative quantity of weak acid required appears to be not noticeablydependent on the ionization constant of the weak acid. In general, anyquantity of weak acid will have some effect, but marked improvement isnot obtained with less than about one equivalent of weak acid to 38equivalents of strong acid. The improvement is increased as the quantityof weak acid is increased up to about one equivalent of weak acid toabout 18 parts of strong acid. Greater relative amounts of weak acid donot produce an efl'ect'commensu'rate with the quantity added and thratio of 1 to 18 appears to be about the optimum.

The weak acid may be either an inorganic acid such as phosphoric or anorganic acid such as acetic, tartaric or citric.

The invention is of particular importance in part. More concentratedsilica-alumina hydrogels are obtained as the ratio of sodium aluminateto an aluminum salt is increased.

The weak acid may, if desired, be supplied wholly or in part as asoluble metal salt to become effective when mixed with the strong acidto prepare the colloidal gelable solution. Thus sodium acetate may bedissolved in water glass and this solution is mixed with sulfuric acidalone in order to achieve the same eflect as adding water glass to amixture of sulfuric and acetic acids.

According to a preferred embodiment of the invention, the gel isprepared in the form of spheroidal pellets.

This process of forming the pellets involves continuously contactingwithin an enclosed mixing chamber, such as an injector or nozzle mixer,streams of reactant solutions of such concentrations and proportionsthat no gelation occurs within the mixer, but only at some predeterminedtime after leaving the mixer, and under suchconditions of flow that eachstream is completely and uniformly dispersed within and throughout theother at the instan of contact. The resulting colloidal solution isejected from the mixer through orifice or orifices of suitable size soas to form globules of the solution which are introduced into a fluidmedium where the globules of the colloidal solution set to a gel beforethey pass'o'ut of that medium. The fluid medium may be any liquid orcombination of liquids which is immiscible with water, such as, forexample, petroleum naphtha, kerosene, hydrocarbon oils, etc.

There are two alternative methods of operation which are dependent uponthe density of the fluid employed. When th density of the fluid is lowerthan that of water, the fluid is supported over a layer of water andthecolloidal solution from the mixer is introduced at the top of the columnof fluid; the height of the latter and the gelation time being adjustedso that gelation occurs within the fluid and before the globoseparticles reach the water surface. For a fluid more dense than water,theprocedure is reversed; the colloidal solution is ejected into the bottomof assasro gel and pass into a layer of water which conducts the gelaway from processing.

The shapes of the formed gel are dependent upon the rate at which theglobules of the colloidal solution travel through the water-immiscibleliquid: while the rate of movement of the globules depends upon therelative density and viscosity of the fluid medium employed. It thelatter medium has a low viscosity and a density far removed from that ofthe colloidal solution, the globules of the latter solution will travelrapidly, hence, the gel pellets will assume flat or disc-like shapes.Examples of liquids in which pellets of this type may be Produced arebenzene, carbon tetrachloride, or petroleum naphtha. A water-immisciblefluid medium having high viscosity or a density close to that of thecolloidal solution will eflect slow movement of the globules of thelatter solution and thus form sphericallyshaped gel pellets. It isapparent from the above description that gell pellets of any shape,varying from flat-like discs to perfect spheres, may be manufactured bychoice of water-immiscible fluid medium having the proper density andviscosity.

A number of variations in the methods described above may be employedwhich are to be considered within the scope of this invention. Forexample, it may be desired to mix the reactant solutions at suchconcentrations that gelation occurs, say, at one minute after thesolution leaves th mixing chamber. This would require a rather longcolumn of fluid medium in order that gelation takes place in the fluid,but a considerably shorter colum'nmay .be used by simply increasing thetemperature ofthe fluid medium so that the time of gelation isdecreased.

Suitable apparatus for the practice of this preferred process is shownin the annexed drawing 49 wherein:

of the apparatus;

Figure 1 is a section through a preferred form Figure 2 shows a modifiedtype of a mixing nozzle;'

Figure 3 is an illustration of a very simple mixing nozzle; and Figure 4is a view of a modifled form of apparatus according to the invention.

Referring to Figure 1, a mixing nozzle indicated generally at It, ismounted at the top of a column of water-immiscible fluid in a tank ll.At the bottom of tank II is' a layer of water which forms an interface Hwith the column of said fluid. Water is. continuously supplied throughinlet II and withdrawn through outlet II. The interface at I! ismaintained by properly adjusting the height of conduit s-in correlationwith the density of the fluid medium and the rate at which water issupplied at it. Vent It prevents siphoning action. The flow of watercarries away the gel pellets through outlets l4 and ate suitable washingand treating stages. The water inwhich the pellets are carried away isitself a washing medium and may include any desired treating material toact as a treating stage.

The colloidal solution from which the pellets are formed is made up andadmitted to the column of fluid by the mixing nozzle Ill. Preferably.the apparatus will include a plurality of nozzles ill in order toincrease the capacity of the unit; but only one is shown here forpurposes of simplicity. The nozzle Ill includes means for completelydispersing two solutions in each other and the fluid. the globules riseup through the fluid, 7s admitting a continuous stream of the so-formedcolloidal solution below the surface IQ of the water-immiscible fluid,wherein the stream of the colloidal solution breaks up into globules.The colloidal solution or globules thereof may be dropped on the surfaceof the fluid but this tends to break them and impairs control overpellet size obtained by injecting the colloidal solution under thesurface of the liquid. It must be borne in mind, that considerableshrinkage takes place, not only by syneresis, but also during drying andprocessing. Control of globule size must take into account thisshrinkage.

The size of the globules is controlled by the rate at which thecolloidal solution flows through the nozzle orifice and the dimensionsof the latter. A simple modification in controlling thesize of theglobules is the introduction of a baffle just outside of the nozzlemixer and in the stream of the colloidal solution. Furthermore, sizingis a matter of relative densities and viscosities of the colloidalsolution and water-immiscible liquid.

In the mixing nozzle l0, solutions to be mixed are metered accuratelyand then admitted through lines I! and is to a chamber which has a rotorls rotated by shaft 20 at a speed of at least about 1700 R. P. M., froma source of power not shown. The rotor I9 is constructed from arectangular bar of metal whose edges are rounded off in such manner thatthe walls of the mixing chamber serve as a guide for them. The roundededges of the rotor are grooved; thus, efficient dispersion of bothsolutions in each other is maintained and gel formation is prevented inthe mixing nozzle. The rotor may be fluted in any suitable manner orprovided with other inequalities of surface to increase agitation in themixing zone. Helical grooves for such purpose are shown on the rotor 2|of the modified form of mixing nozzle illustrated diagrammatically inFigure 2. The best operation of the mixing nozzle is realized when therates of the reactant solutions are so high that the time the lattersolutions spend in the mixing chamber is only a very small fraction ofthe gelatin time.

A further modification is the extremely simple mixer of Figure 3,wherein the rotor 22 is merely a shaft which may be fluted, grooved.etc.

Another modification that may be applied to any of the mixing nozzlesillustrated in Figures 1, 2 and 3 is to provide means for injecting airinto the solutions admitted to the mixing chamber or to the mixingnozzle itself. By this means, hydrogel pellets are obtained whichcontain numerour small bubbles of air which serve to make the processeddry gel less dense in nature and more pOl'OUS.

The apparatus of Figure 4 is adapted for upward flow of the colloidalsolution during gelation. In this case, the mixing nozzle I ispositioned at the bottom of shell I l which contains a column ofwaterimmiscible liquid heavier than water, with water therea'bove, theliquid-liquid interface being again indicated at l2. Water is admittedby a pipe 23 while water carrying gelled spheroids is withdrawn bydischarge line 24.

A peculiar feature of the present gel pellets is their transparency;they having the appearance of clear glass beads, in many cases. Thisappearance is retained only when silica is predominant, the transparencybeing lost as content of other oxides is increased. For example, 12.5%alumina is about the upper limit for glassy appearance of silica-aluminagels, when prepared from colloidal solutions having a pH below 8.

The present pellets are extremely hard; due to this property and theirsmooth surfaces, they are capable of resisting losses by attrition andshock in handling for a period many times longer than the molded pelletsused heretofore.

Example I and 0.066 gram NaaO per cc. A second solution was prepared bydissolving 387 grams of sodium aluminate in water to form liters ofsolution. These two solutions were mixed in batch fashion with eflicientstirring in the ratio of 100 volumes of the former to 19 volumes of thelatter. The sodium silicate-sodium aluminate solution was mixed with4.535 normal sulfuric acid in the ratio of 119.0 volumes ofsilicate-aluminate solution to 47.35 volumes of sulfuric acid in thenozzle mixer. The resulting sol leaving the mixer was introduced intothe top of a column of mineral oil whose depth was eight feet. Theglobules of solution fell through the oil and gelled before passing intothe layer of water located beneath the oil. The/gel in the globular formwas conducted out of the bottom of the column in a stream of water andon removal from the water, it was washed with an aqueous solution of awetting agent to remove oil from its surface. It was then washed withwater and NHiGl solution to replace zeolitically-held sodium ions byammonium ions which are capable of being driven ofi as NH; gas byheating. The gel was dried slowly and uniformly at F. until shrinkagewas substantially complete and the drying was continued at graduallyincreasing temperature up to 1050 F. at which temperature it wasmaintained for two hours. The silica-alumina gel retained its spheroidalshape during the washing and drying operations.

The time of gelation for the concentration and proportions given abovewas 15 seconds, while the pH was 5.5. The concentration of silicaaluminais 13 grams per 100 cc. of the mixed solutions.

Example II-Silica alumina gel This example illustrates the advantage ofa mixture of a weak acid and a strong acid in preparing a silicahydrogel containing" a minor amount of alumina. The concentration ofsilica alumina is 15 grams per 100 cc. of the mixed solution in contrastto 13 grams per 100 cc. as shown in Example I.

An acid solution was prepared by mixing 11.10 liters of 9.2 normalsulfuric acid with 2.00 liters of 3.04 normal acetic acid. This acidsolution was mixed in the nozzle mixer with the sodium aluminate-sodiumsilicate solution prepared as described in Example I. The two solutionswere mixed in the ratio of 26.20 volumes of the former solution to 119.0volumes of the latter. The spheroidal hydrogel pellets were processed bythe method described in Example I.

The time of gelation for the concentration and proportion of reactantsgiven above was 25 seconds. while the pH was 5.5.

Example III-Silica gel The sodium silicate solution prepared asdescribed in Example I was mixed in the nozzle mixer with a 4.910 normalsolution of sulfuric acid in the ratio of 100 volumes of the former to66.9 volumes of the latter. The spheroidal hydrogel pellets wereprocessed as above. The

time ofgelation for the concentration and proportion of reactants wasseconds, while the pH was 5.8. The concentration of silica in sol was 13grams per 100 cc.

Example IV--Silica ael mixed solutions while that of Example III was 13grams per 100 cc.

An acid solution was prepared by mixing 10.85

' liters of 9.2 normal sulfuric acid with 2.00 liters of 3.04 normalacetic. This acid solution was mixed in.the nozzle mixer with the waterglass solution prepared as described in Example I in the ratio of 25.70volumes of the acid solution to 100.0 volumes of the silicate solution.The time of gelation was 10 seconds, while the pH was 5.8.

The spherical pellets of Example I have been compared by hardness teststo pellets formed in the conventional manner. A comparison on crackingefliciency shows the present pellets to have substantially the sameeffect as molded pellets and broken fragments. A silica-alumina hydrogelwas prepared by mixing reagents of the same concentration and in thesame proportions as in Example I. This was .permitted to gel as a massin conventional manner.

The hydrogel, after being washed, was divided into two portions, the onepart was dried, then crushed to produce fragmentary pieces of thedesired size; the other portion of the hydrogel was cast into molds anddried, thus forming small cylindrical pellets. These two forms of gelwere subjected to a hardness test developed for cracking catalysts whichconsists of tumbling an 80 cc. sample of material in a one-pound greasecan with one x 3%" Monel metal rod at 80 R. P. M. on a paint roller millfor a period of one hour, then screening the sample to determine thequantity which has powdered and broken down to a size smaller than theoriginal. The fragmentary pieces of gel showed a breakdown of 12%, whilethe cylindrical pellets were broken down to the extent of 6%. The largerbreakdown with the gel in the fragmentary form is probably due to theirregular shapes and to the stresses and 11ssures developed during thecrushing operation.

The spherically-shaped gel of Example I under the above conditions ofhardness test gave no.

powdering nor breakdown. Continuing the test for an additional 15 hours,merely scratched the surface of the spheres, thus producing only anegligible amount of fines. Subjecting the gel to the hardness test fora total of eighty hours gave 0.3% of material which was smaller in sizethan the original.

The pellets of this invention may act as carriers for other material inthe manner well known in the art. I

In the following claims reference is made to ions of weak andstrong'acids in defining the sol from which the gel is obtained. Theterm "ion" is not limited here to free ions in the sol but includes alsocombined ions, as the combined acetate ion in unionized acetic acid.

We claim:

1. The process which comprises preparing a sol containing silica capableof setting to a hydrogel without substantial formationof gelatinousprecipitate of silica by adding an alkaline silicate solution to an acidsolution comprising a major drogel without substantial formation ofgelatinous precipitate of silica by adding an alkaline silicate solutioncontaining a soluble metal aluminate to an acid solution comprising a.major proportion of a strong acid and a minor proportion of a weak'acid.

3. The process which comprises preparing a sol containing silica capableof setting to a hydrogel without substantial formation of gelatinousprecipitate of silica by adding an alkaline silicate solution to an acidsolution comprising a strong acid and a weak acid.

4. The process which comprises preparing a sol containing silica capableof setting .to a hydrogel without substantial formation of gelatinousprecipitate of silica by adding analkaline silicate solution, containinga soluble metal aluminate, to an acid solution comprising a strong acidand aweak acid.

5. The process which comprises preparing a sol containing silica capableof setting to a hydrogel without substantial formation of gelatinousprecipitate of silica by adding an alkaline silicate solution to an acidsolution comprising a strong acid and a weak acid in a ratio of at leastone equivalent of 1 weak acid to 36 equivalents of strong acid.

6. The process which comprises preparing a sol containing silica capableof setting to a hydrogel without substantial formation of gelatinousprecipitate of silica by addingan alkaline silicate solution, containinga soluble metal aluminate, to an acid solution comprising a strong acidand a weak acid in a ratio of at least one equivalent of weak acid to 36equivalents of strong acid.

7. The process which comprisesvpreparing a sol containing silica capableof setting to a hydrogel without substantial formation of gelatinousprecipitate of silica by adding an alkaline silicate solution to an acidsolution comprising a strong acid and a weak acid in a ratioof about oneto two equivalents of weak acid to about 36 equivalents of strong acid.I

' 8. The process which comprises preparing a sol containing silicacapable of setting to a hydrogel without substantial formation ofgelatinous precipitate of silica by adding an alkaline silicatesolution, containing a soluble metal aluminate, to an acid solutioncomprising a strong acid and a weak acid in a ratio of about one-to twoequivalents of weak acidto about 36 equivalents of strong acid.

9. The process which comprises preparing a sol containing silica capableof setting to a hydrogel without substantial formation of gelatinousprecipitate of silica by adding an alkaline silicate solution to an acidsolution comprising a strong acid and a weak acid in a ratio of aboutone equivalent of weak acid to 18 equivalents of strong acid.

10. The process which comprises preparing a sol containing silicacapable of setting to a hydrosol containing silica capable of setting toa hydrogel without substantial formation of gelatinous precipitate ofsilica by adding an alkaline silicate solution to an acid solutioncomprising a strong acid and a weak acid in a ratio of at least oneequivalent of weak acid to 36 equivalents of strong acid to give a solhaving a pH ofabout 5 to 8. 7

12. The process which comprises preparing a sol containing silicacapable of setting to a hydrogel without substantial formation ofgelatinous precipitate of silica by adding an alkaline silicatesolution, containing a soluble metal aluminate, to an acid solutioncomprising a strong acid and a weak acid in a ratio of at least oneequivalent of weak acid to 36 equivalents of strong acid to give a solhaving a pH of about 5 to 8.

13. The process which comprises preparing a Patentlio. 2,586,810.

111mm n. marsrc, ET AL.

sol containing silica capable of setting to a hydrogel withoutsubstantial formation of gelatinous precipitate of silica by adding analkaline silicate solution to an acid solution comprising a strong acidand a; weak acid in a ratio of about one equivalent of weak acid to 18equivalentsof strong acid to give a sol having a pH of about 5 to 8.

14. The process which comprises preparing a sol containing silicacapable of setting to a. hydrogel without substantial formation ofgelatinous precipitate of silica by adding an alkaline silicatesolution, containing a soluble metal aluminate, to an acid solutioncomprising a strong acid and a weak acid in a ratio of about oneequivalent of weak acid to 18 equivalents of strong acid to give a solhaving a pH of about 5 to 8.

' MILTON M. MARISIC.

SHELDON DRAY.

October 16, 9 45 It. is hereby certified that error appears in theprinted specification of the above numbered patent requiring correctionas follows:' Page 3, first column, linehl for "gelatin? reed--gelation--;-

and second column, line 15, for "100 read --10--; and that the saidLetters Patent should be read with this correction therein that the samemay conform to the record of the case in the Patent Office.

Signed and sealed this 9th day of April, A. D. 1911.60

(Seal) Leslie Frazer 'First Assistant Commissioner of Patents.

